Having the service history for your vehicle can make a huge difference when it comes to selling the car, especially if it has been serviced regularly and by a main dealer. Service history is not just having the service manual but also keeping any receipts for work you have had done.
If you have lost the service book for your car then you can often buy a replacement but it will then be up to you to track down all the garages that have services the car and ask them to stamp the new book again to prove that it has been done. If the car is a number of years old, the garage that serviced the vehicle may not even still be trading meaning you may have to miss out a stamp.
If you have brought a second hand car, you can also contact a garage that you know serviced it and ask for them to stamp a new book so that when you come to sell it you have the information already completed.